1. Field of the Invention
The invention relates to radar systems and more particularly to moving target indication (MTI) radar systems which separate moving targets from stationary ground clutter by utilizing the doppler frequency shift imparted to radar signal returns from moving targets.
2. Description of the Prior Art
Moving target indication (MTI) processing is necessary in air traffic control (ATC) radars to detect the airborne targets in the presence of fixed ground clutter, the echo signals from which are orders of magnitude greater than the echo signals from the airborne targets. The basic MTI concepts were formulated during the massive radar developmental program performed during World War II. Work in this area was continued in post war years and today most modern search radars include some type of MTI processor to detect moving targets in the presence of clutter. The early systems, however, have limited sub-clutter visibility, i.e., the target signal level below the clutter level with which the target may be detected. These radars exhibit limited clutter cancellation, reporting unwanted targets due to ground clutter returns, and are expensive to operate and maintain. By exploiting today's digital technology, major improvements in MTI performance may be obtained economically and reliably.
In many modern MTI radar systems, a limiter is coupled between the i.f. amplifier and the MTI processor to reduce the clutter cancellation burden imposed on the processor. The limit is set to attenuate the clutter from large ground returns, such as mountains, by 20 or 30 dB, while return signals from targets of interest are well below the limiting level. The signal from the limiter is then coupled to the MTI processor in which the clutter signals due to large ground echoes are further reduced to provide residues at the output terminals of the MTI processor that are comparable with receiver noise. Unfortunately, the non-linear action of the limiter causes the spectrum of strong clutter returns to be spread into the passbands of the MTI canceller. This clutter residue, which is generally highly correlated over the pulses that occupy a beam width of data, causes greatly reduced target sensitivities and greatly increased false alarm rates.
The present invention is directed to an improved MTI radar wherein the effects of hard limited clutter are significantly reduced thereby decreasing the false alarm rate and providing increased detection sensitivity to moving airborne targets.